Please see my answers below:
1. Can I add additional VPNs from separate SSIDs to the same MX84?
Yes, it is configured per SSID basis so you can add more SSIDs if you want to be tunneled to the same MX84
2. If I have a second MX84 in a second DC, can i achieve failover between the two?
Yes but it requires manual failover. As you can see on the access control page, you can only select one concentrator at a time so you have to update the configuration to be pointed to the secondary MX.
hope this helps.
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